In the manufacture of UV/EB cured coatings, there are many circumstances in which it is desirable to soften or plasticize the resulting coating to improve its resistance to delamination from a flexible surface. This is particularly true of UV/EB cured printing inks, where delamination can diminish the image quality and UV/EB cured silicone release layers, where lost portions from delamination can clog printer mechanisms, interfering with subsequent printing operations, or more importantly, can result in poor or no release from pressure sensitive adhesives.
Release layers, including silicone release layers, have long been used to protect the adhesive layers for tapes, labels, wall coverings, and other tacky substances. Cationic silicone release layers are commonly prepared from UV or EB curable compositions which can contain epoxy or vinyl ether functional siloxanes, including monomers, oligomers and/or polymers, and cationic photoinitiators that respond to UV light and/or an electron beam (EB). Discussions of epoxy functional siloxanes and cationic photoinitiators used to prepare silicone release layers are set forth in U.S. Pat. Nos. 4,279,717; 5,583,185; 5,500,300; 5,614,640; and 5,814,679; which are incorporated herein by reference for the disclosure of epoxy functional siloxanes cationic photoinitiators therein and the use thereof.
Ultraviolet light cured inks have found use in various printing processes. Most comprise a reactive monomer and/or oligomer, a photoinitiator, one or more a pigments and optional additives. Examples of cationically initiated ultraviolet light curable inks are described in U.S. Pat. Nos. 5,391,685, and 5,889,084, which are incorporated herein by reference for the ink formulations disclosed therein and use thereof.
Of the various types of UV/EB curable inks which are known, many find use in screen printing, overprint varnishes, flexographic printing and ink jet printing methods, examples being those described in U.S. Pat. No. 5,200,438, discussed above, as well as U.S. Pat. Nos. 5,391,685, 4,680,368 and 5,500,040.
Cationic photoinitiators generate strong acids on exposure to UV light or an electron beam (EB). The acids will initiate a chain reaction or polymerization of epoxy (oxirane), oxetane or vinyl ether functional groups.
UV/EB cured coatings, including release layers and print from UV/EB cured inks, can be very brittle, making them very susceptible to mechanical damage. Damage to release layers and print can occur during processing and handling. For example, damage to a release layer can occur in the slitting process in the manufacture of labels. This damage is believed to contribute to the buildup of the release layer and/or adhesive on printer print heads when thermal printable sheets with integral labels are used. In another example, damage to print (cracking) from UV/EB cured inks can occur during handling of the printed material.
The use of common inert plasticizers, such as adipic acid esters, phthalic acid esters, ricinoleate acid esters, citrates, epoxides, glycerols, glycols, hydrocarbons, chlorinated hydrocarbons, phosphates and the like, to make cationic silicone release layers or print from UV/EB cured inks less brittle can present problems. The inert plasticizers remain mobile in the release layer and print, which makes them subject to loss over time through migration or extraction. Migration or extraction of the plasticizer from the print can diminish the appearance of the printed image. Migration or extraction of the plasticizer from the release layers, including silicone release layers, will lead to embrittlement as they age and can lead to changes in the adhesive performance. This can range from increasing the adhesion of the adhesive to the point where a label will no longer release from the release layer, or deadening adhesion to the point where a label will no longer stick to the target substrate.
It is desirable to provide plasticized UV/EB cured cationic coatings, including release layers and print from UV/EB cured inks, which do not embrittle over time and are stable.